Electric machine and method for dynamically balancing the rotor of said electric machine

ABSTRACT

The present invention relates to an electric machine comprising a stator and a rotor ( 10 ), the said rotor being formed of a rotor body ( 20 ) with a stack of laminations ( 14 ) and placed on a rotor shaft ( 12 ). 
     According to the invention, the rotor comprises at least one cavity ( 38 ) for receiving at least one balance weight ( 48 ) for the dynamic balancing of the said rotor.

The present invention relates to an electric machine and to a method for the dynamic balancing of the rotor of this electric machine.

It relates more particularly to a variable-reluctance synchronous electric machine.

In general, an electric machine comprises a fixed part (stator) and a rotary part (rotor) which are arranged coaxially one inside the other.

In such machines, the rotor exhibits imbalance caused by manufacturing tolerances, mechanical design, the distribution of material, etc.

When this rotor is rotating at high speed, this imbalance generates vibration which may cause the machine to malfunction or even cause damage to the rotor or to the machine.

In addition, this vibration is the source of noise which may detract from the comfort of using this machine.

It is therefore absolutely essential to greatly reduce or eliminate rotor imbalance in order to avoid these vibration phenomena.

In order to do so, provision is made for the rotor to be dynamically balanced according to specifications laid down for each machine (noise level, vibrational frequency, etc.).

It is known practice, notably from document FR 1 341 204, to balance the rotor by removing material. More specifically, in order to counter rotor imbalance, machining is used to drill or mill material away from the body of this rotor.

This balancing through the removal of material has not-insignificant drawbacks.

Specifically, depending on how much material is removed, there is a risk that the mechanical properties of the rotor may be severely impaired.

Furthermore, the metal particles generated by the machining operation are liable to impair the operation of the machine. These particles are attracted by the magnetic parts of the rotor and/or of the stator thereby creating disturbances in the running, or even causing the two parts to jam relative to one another.

The addition of material to the body of this rotor is also known as a solution for balancing a rotor.

The material used to compensate for the imbalance may be a paste, which is malleable at the time of application to the rotor, and which will then harden after application, ensuring that it adheres firmly to the rotor.

Balancing by adding balancing paste is, however, something that is difficult to achieve because the paste, after it has been applied to the rotor, takes time to dry and this time is dependent on external parameters such as the air temperature, the relative humidity, etc.

The rotor therefore has to be placed in a storage site for a highly variable length of time.

This entails complex logistics and increases the cost of production.

The material used may also be calibrated metal weights added to the rotor in order to balance same, such as clips, screws, etc.

By way of example and as best described in Patent Application FR 2 421 498, these metal weights are perforated sheets which are fixed by clamping or bonding onto axial pins emanating from the rotor.

In these variable types of balancing involving the addition of material, it has, however, been found that, in the event of very significant angular acceleration or deceleration of the rotor and according to the ageing of the machine, the paste or the calibrated weights may become detached from the rotor and therefore cause this rotor to become imbalanced. This loss of balancing therefore triggers vibration which is detrimental to the operation of the machine and to the comfort of the user.

In addition, the paste or the calibrated weights which have become detached may impede the rotation of the rotor causing serious machine malfunctions.

These disadvantages are all the more significant in the case of a variable-reluctance electric machine.

Such a machine comprises a rotor bearing permanent magnets which are housed near flux barriers borne by this rotor.

This configuration therefore makes balancing through the removal of material even more difficult given the small volume of material of which the rotor is made and because this removal that needs to be performed in order to achieve balancing needs to be done in a flux barrier which, by definition, is an empty space.

In addition, adding material carries the risk of perturbing the propagation of the magnetic flux of the magnets, notably when this addition is done in one or more flux barriers.

The present invention seeks to overcome the disadvantages listed hereinabove by balancing the rotor using an addition of material without this addition being able to perturb the correct operation of the electric machine.

To this end, the present invention relates to an electric machine comprising a stator and a rotor, the said rotor being formed of a rotor body with a stack of laminations and placed on a rotor shaft, characterized in that the rotor comprises at least one cavity for receiving at least one balance weight for the dynamic balancing of the said rotor.

The cavity may be positioned along the longitudinal axis of the rotor and substantially parallel thereto.

The cavity may be formed by a punching made in the laminations.

The rotor may bear recesses housing magnetic-flux generators and recesses forming flux barriers, and the cavity may be formed near the periphery of the body.

The cavity may be situated between the recess housing the magnetic-flux generator furthest from the rotor shaft and the periphery of the rotor body.

The cavity may have a closed cross section.

The invention also relates to a method for the dynamic balancing of a rotor of an electric machine, the said rotor comprising a rotor body formed by a stack of laminations and placed on a rotor shaft, characterized in that:

-   -   the rotor imbalance is determined,     -   the quantity of balance weight needed to counterbalance the         determined imbalance is determined,     -   at least one balance weight is introduced into at least one         receiving cavity placed in the body of the rotor.

The balance weight may be forcibly introduced into the cavity in order to immobilize it.

The other features and advantages of the invention will now become apparent from reading the description which will follow, which is given solely by illustrative and non-limiting example, and to which are appended:

FIG. 1 which is a schematic view of the rotor of the invention in axial section on 1-1 of FIG. 2, and

FIG. 2 which is a schematic view of the rotor in radial section on 2-2 of FIG. 1.

According to the embodiment illustrated in FIGS. 1 and 2, a rotary electric machine comprises a stator (not depicted) and a rotor 10.

As illustrated in FIG. 1, this rotor comprises, in the way known per se, a shaft 12, preferably magnetic, on which a stack of identical planar ferromagnetic laminations 14 bearing a plurality of magnetic-flux generators 16 is placed.

With reference to FIG. 2, the circular laminations 14 comprise a central bore 18 through which the rotor shaft 12 passes and a plurality of axial recesses which pass right through the laminations.

As is known, the laminations are assembled with one another by making the bores and the recesses align using any known means, such as bonding, pressing, etc.

Thus assembled, the laminations form the body 20 of the rotor 10 which bears the shaft 12 via the central bores 18.

This configuration is more particularly applied to a variable-reluctance electric machine as will be better described later on.

In this configuration, the body comprises a first series of axial recesses which houses magnetic-flux generators, and another series of axial recesses which makes it possible to create magnetic-flux barriers.

The first series of recesses 22 is, in this instance, in the shape of a quadrilateral, in this instance of a rectangle. These recesses 22 accommodate the magnetic-flux generators, in this instance permanent magnets 24 in the form of bars, likewise rectangular, of a length substantially equal to the length of the body. These recesses will be referred to in the remainder of the description as “housings”.

These housings 22 are positioned radially above one another and at a distance from one another measured from the centre 0 of the bore 18.

As best visible in FIG. 2, these rectangular housings 22 are distributed along axes XX′ and YY′ which are substantially orthogonal and pass through the centre O.

In the example of FIG. 2, each half-axis (OX, OX′; OY, OY′) bears three axial housings 22 of which the longest faces are perpendicular to the half-axes and of which the dimensions of these faces decrease from the centre O towards the periphery of the stack of laminations. Likewise the height of these housings becomes smaller from the centre O towards this periphery.

The housing 22 closest to the bore 18 may leave a bridge of material 26 to this bore and a bridge of material 28 remains between each housing.

The housing 22 furthest away from the bore 18 is positioned some distance away from the peripheral edge of the body.

The other series of recesses consists in perforations 30, of substantially constant height h and of inclined radial direction, starting from these housings and arriving near the edge of the laminations.

These perforations start from the lateral edge 32 of the housings 22 and rise, at an angle a with respect to a straight line parallel to the housings 22, to arrive in this vicinity.

As depicted in FIG. 2, the inclined perforations are arranged symmetrically with respect to the housings. More specifically, a series of three inclined perforations is positioned on one side of the half-axis and another series of three inclined perforations is placed on the other side of this same half-axis.

Thus, a geometric figure is formed in each instance that is substantially in the shape of a V with a flattened bottom with the flat bottom being formed by the housing 22 and with the inclined arms of this V being formed by the perforations 30. This then, on each half-axis, yields three V-shapes which are superposed and some distance apart and that have heightwise and widthwise dimensions that decrease from the bore towards the periphery of the body.

Thus, aside from the bridges of material 26, 28 there remains a solid part 34 between the inclined perforations of each V-shape and another solid part 36 between the perforation closest to the bore of one series of three V-shapes and the perforation closest to the bore of another adjacent series of V-shapes.

This body further comprises at least one receiving cavity 38 for receiving rotor balance weights and which is situated between the V-shape furthest from the bore of the body and the periphery of the body of the rotor.

In the example of the figures, there is a cavity placed on each half-axis.

Advantageously, this cavity extends from one lateral face of the body to the other face, substantially parallel to the shaft 12.

This cavity results from a punching operation 40 performed on each lamination and from the assembling of the laminations with one another thereby forming this cavity.

In addition, there are also two perforations 30′ inclined by an angle a which start from the vicinity of the cavity 38, leaving a wall 41 remaining and which end in the vicinity of the peripheral edge of the body, while being symmetric with one another with respect to the half-axis. As a result, the cavity is positioned between the two perforations 30.

This then forms a bridge of material 42 between the cavity and the housing and a solid part 44 between the perforations 30 and 30′.

Flux barriers 46 formed by the perforations are thereby created. The magnetic flux coming from the magnets can therefore pass only through the bridges of material and the solid parts.

The cavity, in the case of the figures, has a closed cross section, in this instance a circular cross section, but any other cross section may be envisaged, such as a polygonal cross section.

This cavity is therefore designed to receive at least one balance weight 48 which is immobilized in this cavity.

This weight may have a cross section similar to that of the cavity but with sizing greater in cross section so as to be able to be immobilized therein.

By way of example, in the case of a cavity of circular cross section, the balance weight may be a ball 50 of a diameter greater than that of the cavity so that this ball is held in the cavity by friction.

In another example, the balance weight may have a cross section which is different from but which complements that of the cavity, such as a bar of polygonal cross section that is forcibly introduced into the cavity of circular cross section.

In order to balance the rotor 10 it is necessary first of all to define and locate the imbalances that need balancing.

This operation is performed on appropriate machines which are widely known to those skilled in the art.

This makes it possible to determine, generally by calculation and in accordance with the imbalance correction laws, the quantity and positioning of balance weights to be added to the rotor in order to balance same.

The method according to the invention therefore consists in adding one or more balance weights, in this instance in the form of balls, to one or more cavities 38.

By virtue of this the rotor can be balanced without weakening it by removing material or perturbing the transmission of the magnetic flux by removing or adding balancing material into the bridges and/or the solid parts. 

1. Electric machine comprising a stator and a rotor, the rotor being formed of a rotor body with a stack of laminations and placed on a rotor shaft, wherein the rotor comprises at least one cavity for receiving at least one balance weight for the dynamic balancing of the rotor.
 2. Electric machine according to claim 1, wherein the cavity is positioned along the longitudinal axis of the rotor and substantially parallel thereto.
 3. Electric machine according to claim 1, wherein the cavity is formed by a punching made in the laminations.
 4. Electric machine according to claim 1, in which the rotor bears recesses housing magnetic-flux generators and recesses forming flux barriers, wherein the cavity is formed near the periphery of the body.
 5. Electric machine according to claim 4, wherein the cavity is situated between the recess housing the magnetic-flux generator furthest from the rotor shaft and the periphery of the rotor body.
 6. Electric machine according to claim 1, wherein the cavity has a closed cross section.
 7. Method for the dynamic balancing of a rotor of an electric machine, the rotor comprising a rotor body formed by a stack of laminations and placed on a rotor shaft, wherein: the rotor imbalance is determined, the quantity of balance weight needed to counterbalance the determined imbalance is determined, at least one balance weight is introduced into at least one receiving cavity placed in the body of the rotor.
 8. Method according to claim 7, wherein the balance weight is forcibly introduced into the cavity in order to immobilize it. 